The invention relates to a device for low temperature refrigerative drying of compressed air and other gases.
The motivation for the invention comes from the desire to provide a refrigerative drying device for air and/or other gases whereby the gas is cooled to several (or a number of) degrees below zero degrees Centrigrade (&lt;0.degree. C.), wherewith the problem of plugging of lines with ice is solved, which plugging occurs when a drying device is employed which lowers the temperature below the critical temperature of 2.degree. C. With the elimination of the plugging problem, the refrigeration of the gas successfully reduces the moisture content of the gas practically to zero.
If one observes a compressed air installation, it is quite clear that when moisture is present in the air there are major and costly consequences, because moisture causes oxidation and consequent degradation in the interior of the tubing, and attacks the pneumatic components (i.e., the components of the gas handling equipment, e.g., air cylinders or other actuators, instruments, etc.), with adverse effects on their performance. In a number of processes, moisture in the air will contaminate the product, and/or will increase costs due to interruptions and loss of production.
Liquid water develops in the tubes of compressed air equipment by condensation of the water vapor contained in the air, which condensation is brought about by natural decreases in ambient temperature. If, at a point disposed upstream of the point at which the compressed air enters the consuming network, the air is maximally refrigerated (i.e. to approximately 2.degree. C.), and the water which condenses is removed by automatic drainage, one obtains air which is dry and is at its normal condition of use.
In current compressed air equipment of conventional types, moisture enters the equipment through an air/air heat exchanger, where it is pre-cooled, wherewith the condensation of water vapor begins. The air then passes to the "evaporator", which is an air/refrigerant gas heat exchanger, at the exit of which the air reaches its lower temperature, typically 2.degree. C. It is cooled to the extent that most of the humidity which it contains is condensed. The condensed moisture is collected in a separator, where it is expelled through an automatic drainage trap. Before this cold, dry air passes into the distribution network it is passed through the (e.g. countercurrent) primary heat exchanger, where it serves to pre-cool the intake air. This heat exchanger saves power which might be consumed in cooling the intake air, and provides heat to the compressed air which avoids condensation on the exterior of the downstream compressed air lines. The compressed air processed in this manner contains approximately 0.66 grams water vapor per cubic meter of air.
The described conventional equipment meets the normal requirements of industry as long as there are no special conditions which necessitate a lower final moisture content in the air.
In order to achieve a lower final moisture content by means of the refrigeration system, the air must be compressed at &lt;0.degree. C., rather than the presently employed +2.degree. C. However, when such lower temperature compression is attempted the water vapor is first condensed and then it begins to freeze, causing blockage of the tubes of the above-described primary heat exchanger due to the presence of ice, thereby eventually causing interruption of the process.